Refractory compound materials find many applications in various areas such as automobile, metallurgy, electronics and chemical engineering due to their unique processing characteristics and performance properties. In the case of nitrides, boron nitride (BN) can be used as high temperature lubricants, cutting tools and crucibles; titanium nitride (TiN) is an extremely hard ceramic material, and often used for coatings; aluminum nitride (AlN) is a substrate materials for semiconductor devices and a desired heat spreading material for optoelectronic packaging; and gallium nitride (GaN) based alloys have been the working horse for making blue ray laser and high brightness LED devices, and the latter is poised to revolutionize the lighting industry. This type of examples can keep going for a long list.
Synthesis and processing of refractory compound materials are among the most challenging technologies in current chemical engineering and material science. The conventional crucible melting, casting and subsequent machining practices have been proven impossible for this kind of materials due to their ultrahigh melting points, supreme hardness and low ductility. While powder metallurgy has been proven to be feasible for components fabricated with refractory compounds, the synthesis of powders have been found very time consuming and expensive. Methods for making refractory compound powders include chemical reduction, grinding, Sol-Gel process and carbon thermal reduction etc. Besides the expensive price, the refractory compound powders produced with these methods suffer serious contamination from source materials, process tools and containers.
The gas atomization approach has been widely used for producing metal powders, and this method utilizes high pressure air, nitrogen or argon as spraying medium to break down metal melt stream into melt droplets. In this method, atomization gas traveling in ultrasonic speed is sometimes required to minimize the liquid metal droplet size, and there are also requirements on overheating the liquid source metals to minimize the metal droplet surface tension, sometimes up to temperatures of 2-3 times of their melting points expressed in the unit of Kelvin. Due to the excellent heat exchanging conditions and the fine droplet size, the cooling rate for these droplets can reach levels of 100˜10000K/sec. This cooling rate is orders higher than that of casting ingots. As results of this high cooling rate, the atomized powder has very uniform composition and microstructure, and the materials made of this type of powders have superior performance without segregation. Compared to other production methods, the gas atomization has characteristics of low energy consumption, high production efficiency, high purity, fine powder size and easily be adapted to industry scale. Also, almost all metals can be transformed into powders with this atomization method.
While gas atomization has been widely used for producing metal powders, and a few studies show this method can be used for producing thin compound coating layers on the surface of metal powders for improved environmental stability as reported in U.S. Pat. No. 5,073,409 (Dec. 17, 1991), U.S. Pat. No. 5,372,629 (Dec. 13, 1994), U.S. Pat. No. 5,589,199 (Dec. 31, 1996), U.S. Pat. No. 5,811,187 (Sep. 22, 1998) and U.S. Pat. No. 6,444,009 B1 (Sep. 3, 2002)) etc., there are very few successful efforts reported to synthesize refractory compound powders with this method due to exceptionally high melting points and the tendency to dissociate at that high temperature as well as the electrical insulation properties of the refractory compound materials. There is simply no suitable method to melt the source refractory compounds into superheated liquids.
In this invention, a gas atomization method is proposed to produce high purity refractory metal powders in a cost-effective way with fast production rate. In this invention, the reaction chamber is first pumped to ultrahigh vacuum, and then elementary metal melt is atomized into fine droplets and react with reactive gas to form the refractory compound powders. To make sure the metal melt completely transfer into compound powders, a mechanism is proposed to increase the reaction time between metal droplets and the reaction gas. Characteristics of this method include high purity, fine powder size, fast production rate and energy efficient as well as minimized impact to environment.